Which set of ordered pairs represents a function?
{(-6,-4), (1, -7), (-4, 3), (-4, 8)}
{(-8, -3), (-8, 9), (9, 6), (1, -4)}
{(4,1), (-6, 6), (5,-2), (-4,1)}
{(-3, 6), (-1,0), (-8, 1), (-8, -3)}